Synchronized pulse generator



M. M. LEVY SYNCHRONIZED PULSE GENERATOR Feb. 22, 1949.

File? March 15, 1944 2 Sheets-Sheet 1 my EI'WOIPK F/(i Za,

Feb. 22, 1949, M. M. LE VY 2,462,109 SYNCHRONIZED PULSE GENERATOR Filed March 15, 1944 2 Sheets-Sheet 2 Inventor- (0/ Attorney Patented Feb. 22, 1949 SYNCHRONIZED PULSE GENERATOR Maurice Mo'i'se Levy, London, England, assignor,

by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a

corporation of Delaware Application March 15, 1944, Serial No. 526,605 In Great Britain April 28, 1943 3 Claims.

The present invention relates to the improvement of electric relaxation oscillators, multivibrators, and like generators of periodic wave forms, and in particular to improved frequency dividing arrangements in which a periodic wave generator is synchronized on a submultiple of a controlling frequency.

It is well known that multivibrators and similar devices are liable to be unstable when synchronized on a submultiple frequency, particularly when the submultiple is of high order. They tend to jump from one frequency to another as a result of small changes in the operating conditions. For this reason it has hitherto not been practicable to use in frequency dividers a stepdown ratio greater than about 3 in one stage; and this often necessitates the use of a large number of stages in tandem.

Various arrangements have been proposed for improving the stability of multivibrators; for example, feedback arrangements through resonant circuits tuned to the submultiple frequency; but such arrangements are still found to be limited to a step-down ratio of about 10. The present invention enables the step-down ratio to be still further increased, and the circuit does not become more difl'icult to synchronize as the stepdown ratio becomes larger.

According to the invention, therefore, there is provided a circuit for generating periodic electric waves adapted to be synchronized at a frequency which is a given submultiple of the frequency of controlling waves applied to the circuit, comprising means for generating unidirectional synchronizing pulses repeated at the given submultiple frequency, and means controlled jointly by the said pulses and the controlling waves for generating the said periodic electric waves.

The invention may also consistin a circuit for generating periodic electric waves of saw-tooth form and for synchronizing them on a submultiple of the frequency of a controlling wave, comprising means for deriving from the saw-tooth waves a train of unidirectional synchronizing pulses repeated at the submultiple frequency, and means for applying the pulses and the controlling wave simultaneously to the generating circuit.

In another form the invention provides a multivibrator circuit for generating a train of unidirectional electric pulses repeated at a, given frequency and synchronized on a submultiple of the frequency of a controlling wave, comprising means for deriving from the circuit a train of synchronizing pulses repeated at the given frequency, and means for applying the synchronizing pulses together with the controlling wave to synchronize the multivibrator circuit.

Theinvention will be described with reference to the accompanying drawings in which:

Fig. 1 is a diagram used to explain the principle of the present invention;

Fig. 2 shows a schematic circuit diagram of a relaxation oscillator synchronized according to the invention;

Fig. 3 gives diagrams used in explaining the action of Fig. 2;

Fig. 4 shows a schematic circuit diagram of a multivibrator synchronized according to the invention; and I Fig. 5 is a diagram used to explain the action of Fig. 4.

The method of dealing with the difiiculty according to the present invention is shown in Fig. 1. The negative grid bias voltage provided by the battery B2 is increased so that the striking voltage is also increased. Short duration unidirectional pulses are derived from the saw-tooth waves and are delayed and mixed with the control wave and applied to the terminals l, 2. The sign of the pulses is chosen so that they -momentarily reduce the negative bias applied to the control grid. The effect of the striking voltage curve is shown in Fig. 1. It will be seen that the pulses depress every tenth loop of the control wave so that the upward slope of the sawtooth wave can make contact therewith. If the amplitude of the synchronizing pulses is chosen so that contact is made approximately at the centre of the flank of the controlling wave as shown, that is, when the voltage of the wave is passing through zero, then considerable variation of the operating voltage or amplitude of the controlling wave is allowable'before control is lost, and the adjacent loop of the control Wave is out of the way and therefore cannot intercept the saw-tooth curve. =It is to benoted also that a variation in the amplitude of the controlling wave does not appreciably affect the synchronizing, since the location in time, and height above the time axis, of thecentre point of the flank of the waveremains constant as the amplitude varies, the height above the axis being determined by the height of the pulse.

The duration of the pulse is not of critica importance; it should preferably be about equal to the period of the controlling wave, but it should not be much greater, otherwise part of the adjacent p may be depressed into the path of the saw-tooth wave and may be a source of'.in

stability if, for instance, the slope of the sawtooth curve varies.

It will be evident that since only one particular loop of the controlling wave selected by the pulse can be effective in synchronizing the sawtooth wave, there is no reason for "the stability to get worse as the order of the synchronizing submultiple increases.

As will be explained later, the pulse which causes the discharge of the condenser (31,111 Fig. 2, thereby producing the fiy-back stroke, is derived after a definite delay from the preceding fly-back stroke of the saw-tooth wave. So long as the delay is approximately correct, the desired result .will be obtained, and errors in the time spacing of the pulses do not accumulate.

Fig. 2 shows one arrangement according to the invention, in which the synchronizing is carried out by the method described with reference to Fig. 1. waves is of the relaxation oscillator type and coinprises a gas filled valve V1 having a condenser Cl connected between the anode and the cathode, bias source being represented by the high tension source HT and a resistance RI connected between the anode and the positive terminal of source HT. The control grid of the valve is connected to a grid biasing battery B2 through the secondary winding of the transformer T. Condenser Cl charges up through resistance RI until its potential reaches the striking potential of the valve and is then practically instantaneously discharged until its potential is reduced to the extinguishing potential of the valve. The valve V2 is arranged as a simple pulse generator which is excited by the saw-tooth waves obtained through condenser C2 from a tapping on the resistance R1. The control grid of V2 is connected to the cathode through two series resistance R2 and R3 and a battery or other source B3 adapted to apply a positive potential to the control grid. The resistance R3 should be a relatively high resistance adapted to ensure that the positive grid potential is small.

The point on R1 to which the condenser C2 is connected should be chosen so that the sudden reduction in the voltage applied to the control grid of V2 which occurs at the fly back stroke of the saw-tooth wave is sufficient to block the valve. The condenser C2 becomes charged at the same time, and discharges through the resistance R2 and the lower portion Rs of R1. If the time constant C2(R2+Rs) is small compared with the time constant C1-R1 of the relaxation circuit, then the grid potential of V2 will quickly rise until the valve is unblocked and anode current begins again to flow. Thus the anode potential of V2 suddenly rises to a maximum on the occurrence of the flyback stroke, and then remains constant for a short period determined by the time constant 02- (Rz-i-Rs'), and then falls to the original value again, producing very nearly rectangular pulses across the anode resistance In Fig. 3 curve 0; shows the saw-tooth waves'and b the voltage pulses obtained across the resistance R4. These pulses are applied through a blocking condenser C3 to the input terminals of the delay network, the output terminals of which are terminated with a resistance R5. The upper terminal of R5 is connected by the conductor :1: back to the control grid of the valve V1 through the biassing battery B2 and the secondary winding of the transformer T.

Delayed pulses will be obtained at the output of the delay network as shown in Fig. 3, curve 0.

' The n pulse of -c is delayed with respect to the 11* pulse of b by a time T1 which is very slightly The circuit for producing the saw-tooth iii 4- shorter than the desired period T of the saw-tooth waves, so that the (n 1W fly-back stroke of the saw-tooth wave occurs approximately at the centre of the n delayed pulse. The delay T1 can be obtained by appropriate design of the delay network.

The duration of the pulses b and 0 can be controlled. by adjusting the time constant Cz- (R2+Rs), which may be done by varying any or all of the components C2, R2 and Re.

It should be mentioned that the battery B3 is not essential and may be omitted. The circuit will operate in this case in very nearly the same way; the grid of V2 will, however, be slightly negative instead of slightly positive in the intervals between the pulses.

This battery may, however, be used to control pulses generated in a different way. In this case the time constant C2'(R2+Rs) should be made very small so that the coupling circuit will not have any appreciable effect on the pulses. The voltage of the battery B3 is chosen so that the flyback stroke of the saw-tooth wave takes the control grid of V2 only slightly beyond the cut-off. The immediately following rising slope of the sawtooth wave will very soon unblock the valve, generating pulses similar to those shown at b in Fig. 3. The duration of these pulses is determined by the voltage of the battery 13;. It will be evident, of course, that any other more convenient type of biassing source may be used in place of the battery 13:.

One preferred form of the delay network consists of an artificial non-dissipative transmission line which may be made up of a number of lowpass filter sections comprising series inductances and shunt condensers, adjacent inductances being preferably mutually coupled. The sections should be chosen to be sufficiently short so that a high cut-off frequency is obtained, in order not appreciably to deform the pulses. Such an artificial line is very convenient because the delay is substantially independent of frequency over a wide range, and if means is provided for tapping off at any section of the filter, a delay adjustable over a wide range may be obtained. If such a delay network is used, the resistances R4 and Rs should be adjusted so that the network is terminated at both ends by its characteristic impedance, in order to avoid reflections of the pulses at the terminals.

The pulses generated by the methods already described will be slightly trapezoidal in form on account of the slope of the trailing edges resulting from the discharge curve of the condenser C2 (or from the upward slope of the saw-tooth waves). By a slightmodification of the arrangement of Fig. 2, truly rectangular pulses can be obtained. In this modification shown in Fig. 2a the conductor 11: is removed, and the connection back to the control grid of V1 is taken instead through'the connection y to an adjustable tap t at some intermediate point of the delay network. The resistance R5 should also be short-circuited as shown. The grid battery or other source .83 should preferably in this case bias the grid negatively as shown in Fig. 2a.

The time constant Cz-(Rz-l-Rs) should be adusted to be rather larger than before, so that the duration of the pulses generated by the valve V2 is not much smaller than the period of the sawtooth wave. Such pulses are shown at d in Fig. 3. These pulses are transmitted through the delay network and reach the tap t after a. time T1 slightly 'less than the period Of the saw-tooth wave, as shown at e. The pulses continue to the short circuited end, where they are reflected with an inversion and return to the tap t after a further short period T2, as shown at 1. They continue back to the input end of the delay network where they are absorbed without reflection, the network being properly terminated by R4 as already explained.

Curve 9 of Fig. 3 shows the resultant eifect at the tap 1?. Short rectangular upwardly directed pulses are obtained by the superposition of the two series of pulses e and 7, each pulse being preceded by a downwardly directed V-shaped pulse. This latter pulse could be removed by amplitude limitation or in any other way if desired, but its presence is not harmful, since the effect will be only to raise momentarily the striking voltage of the valve V1 at a time when it is not required to strike.

It will be evident from the above explanation that the duration T2 desired for the pulses is obtained by arranging the tap it at a distance from the short-circuited end corresponding to T2/2. The delay period T1 which will be approximately equal to TTz/2 gives the distance between the input terminals of the network and the tap t. In practice it will be found convenient to provide an adjustable short-circuiting switch (not shown) by which the network may be short-circuited at any desired intermediate point. By this means both the times T1 and T2 can easily be selected for a variety of conditions.

In Fig. 2, an output terminal 5 is shown connected to a variable tap on the resistance R1. The synchronized saw-tooth waves may be obtained from this terminal. Alternatively rectangular pulses similarly synchronized may be obtained from the terminal ii connected to the lead by which the pulses are conveyed to the grid of V1.

Although an arrangement in which synchronization is on a submultiple of order 10 has been used for illustration, the invention is applicable to any submultiple. The theoretical lower limit is order 2, but synchronization by known methods is adequate up to about 3. There is no reason for setting an upper limit to the order of the synchronizing submultiple except that the time T1 will increase with the order and thus increases the bulk of the delay network, so that by the time the 40 order, for example, is reached, this network may have become rather cumbersome. The stability of the arrangement, however, does not become appreciably worse as the order increases. It should be emphasized that while by any of the known methods the limit is about the 10 order,

' the present invention is practicable and convenient at least up to the 30 order.

The particular arrangement of Fig. 2 which has been described as an example, is however, only one rather simple form of the invention. In essentials, it comprises a relaxation oscillator represented by the valve V1, a pulse generator represented by the valve V2 and means represented by the delay network for obtaining delayed pulses which are fed back to the relaxation oscillator together with the controlling Wave, and are applied to some element of that oscillator whereby its frequency may be controlled and synchronized on the desired submultiple of the controlling frequency.

The same principles may also be applied to synchronizing a multivibrator circuit on a submultiple of the controlling frequency. In Fig. 4

is shown a modification of one well known form of a multivibrator comprising two valves V3 and V4 having their control grids and anodes crossconnected through condenser-resistance circuits 07, R1 and Cs, R8. The resistances P are appropriate anode current supply resistances, and the grids are negatively biassed by suitable means represented by batteries B5 and B6.

Connected in series with the resistance Ra (which is the one which is concerned in defining the frequency of repetition) is the secondary winding of a transformer T through which the controlling wave is applied, and a resistance R9 shunted by two delay networks DNI and DN2. These networks are used for generating synchronizing pulses which operate similarly to those previously described. Ignoring these pulses for the moment, it will be understood that the synchronizing wave will be superposed as ripples on the dotted curve (1, and every third ripple, for example, will cross the line Vc, thereby determining the time at which the next pulse is emitted. Since, however, the slope of the curve d is so small, the arrangement will be unstable for submultiples of high order. If, however, the synchronizing ripple is superposed on the top of a pulse, it will be raised up out of the way of the preceding ripples and the arrangement becomes stable. This can be seen from Fig. 5 which shows when the synchronizing Wave and pulse are supplied. In Fig. 5, the full line represents the actual variation of the grid voltage.

The negative grid bias voltage produced by B6 is first of all increased, so that the dotted discharge curve at would occupy the position shown in the absence of the synchronizing Waves. The leading edge 1 of the synchronizing pulse is timed to occur slightly before the time when the generated pulse is to be emitted, and its duration should be about equal to one period of the synchronizing Wave. One loop of the wave is thus raised up so that it cuts the line V0 causing the generated pulse to be emitted. The leading edge of this pulse is shown at a. As described in connection with Fig. 2, the pulse amplitude should preferably be chosen so that synchronizing occurs at the centre of the flank of the wave.

It will be evident that the synchronizing pulse and waves will be superposed on the grid voltage pulse of which a is the leading edge. If desired, therefore, it may be arranged so that the amplitude of this grid voltage pulse is sufliciently great to ensure that after the synchronizing pulse has disappeared, the superposed ripples do not bring the grid voltage below the line Va at any time before the emitted pulse disappears, so that this emitted pulse may not be distorted by the synchronizing wave.

The synchronizing pulse for the (n+1) pulse is obtained from the n pulse by reflection in the delay network DNl whose output terminals are left open as shown. In this way the n pulse which is generated across the resistance R9, is reflected back without an inversion, as desired. The total delay suffered by the reflected pulse in DNI should be a little less than the repetition period T desired for the generated pulses. The network DN2 has its output terminals shortcircuited in order to obtain an inverted reflected pulse delayed slightly longer than the pulse reflected in DNl, so that the combination of the two reflected pulses may produce a short synchronizing pulse in a manner similar to that described with references to the curves d to g of aecaroe.

Fig. If thedurationzof'thesynchronizingpulse- It may be added that the network DN2 is not.

essential'and can beomitted, in which case the synchronized pulse will have the same duration as the emitted pulse. 1 This will operatesatisfactorilysince the synchronizing pulse will have disappearedbefore the emitted pulse is due to dis:- appear.--::

It is to:be noted that the resistance R9 should bexchosen so that the networks are correctly terminated; in order to avoid any further reflections of-thepulses at the input terminals of the networks.

In theembodiments of the invention which have been described, each cycle of thegenerated wave is synchronized by a pulse derived from the, immediately preceding cycle. synchronizing pulse could have been derivedfrom any other previous cycle by appropriately increasingthe delay introduced by the networks.

Theiollowing characteristics of the synchronizing-arrangement of the invention may be, noted:

1. .The delay network determines at which submultiple of the controlfrequency the relaxationaoscillator or multivibrator device is to oscillate-{v Aslthe'delay canbe very precisely chosen and is substantially invariable; the arrangement is verystable anduis practically unaffected by variation of .the operatingconditions. Either it oscillates at the chosen submultiple frequency, ornot at all. In theory any submultiple may be chosen, the practical limit being determined by the physical dimensions of the network, and being probably somewhere about the 40 order.

2. From Fig. 1 it canbe seen that synchronizingwill occur so long as the upward slope of the sawtooth wave meets the synchronizing wave somewhere on the descending flank This means that for a given setting of the delay network, the: control frequency fc may vary vwithin the limits where n is the order of the submultiple employed. Thus the. allowable variation for fc is i2 for the 10 submultiple and il /1%, for tho 20 'submultiple.

Actually the 3. The range of synchronization is practically independent of the amplitude of the synchronizing wave, so that the circuit does not have to be readjusted if the control voltage changes. In known arrangements the synchronizing range voltages and control voltage which do not sub stantially affect the shape of the waves can vary within wide limits which are independent of the order of the submultiple employed;

5. The parameters-which do aifectthe wave shape, such'as the values of the timing resistances-5 andcondenserscan also vary within wide limits,

which however are inversely proportional to the order of the submultiple employed.

The multivibrator described with reference to 4 Fig. 4 is a simplified form of only one possible type to which the invention is applicableuThesame principles maybe applied in numerous other like cases.

What. is claimed is:

1. A circuit for generating periodic electric waves of sawtooth form and for synchronizing them in a submultiple of the frequency of a con-.

trolling waveacomprising a relaxation oscillator forgeneratingsaid sawtooth waves, a pulse gene erator .for converting the waves into unidirectional synchronizing pulses. at the submultiple frequency, a delay network connecting the pulse generator to the oscillator for applyin the pulses thereto and means. for applying. said controlling waveto said oscillator simultaneously with ,said pulses.

2. An. arrangement forv generating periodic electric waves-of saw-tooth form and for synchronizing them on a sub-multiple ofthe frequency,

ofa controllingwave, a relaxation oscillator for generating the saw-tooth waves said oscillator comprising a gas-filled triode having a control grid, and arranged periodically to discharge a condenser, a pulse generator for deriving from the saw-tooth waves themselves a trainof unidirectional synchronizing pulses repeated atthe sub-multiple frequency, a delay network for delaying the said synchronizing pulses by a time interval slightly shorterthan the period of the saw-tooth waves, and means to apply the pulses from the delay network together with the controlling wave simultaneously to the control grid of said-triode.

3. An arrangement according to claim 2 in which the said delay network is short-circuited at a point remote fromits input terminals and the said delayed pulses are tapped off from a point intermediate the said input terminals .and the said short circuit point.

MAURICE MoisE LEVY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Wheeler Aug. 20, 1940 larnett Aug. 20, 1940 White 1 Aug. 27, 1940 Wilson Nov. 12,. 1940 FOREIGN PATENTS Number Country Date VVilsol'l Feb. 6, 1940' Blui nlein Dec. 16, 1941 1 Great Britain Feb. 14, 1938 

